1. Field of the Invention
This invention is directed to treating fabrics for use in protective apparel with viscous polymer solutions.
2. Description of Related Art
Current soft body armors made from woven fabrics require high area weight density, partly in order to achieve less than 44 mm Back Face Deformation (BFD as required by NIJ standard 0101.04 Revision A). BFD is an indicator of blunt trauma, the lower the BFD, the better the protection from blunt trauma. Although many soft body armor constructions can adequately stop ballistic projectiles, the shock associated with blunt trauma can still cause substantial injury or death. Because woven fabrics and the related soft body armor made therefrom typically exhibit high BFD values, higher basis weight are often required for compliance with NIJ standard 0101.04 rev. A. For example, current 100% woven Kevlar® vests weigh more than 1 pound per square foot (psf) for level II protection under the NIJ standard. For example, conventional fabrics are often impregnated with solid adhesives, such as polyethylene laminated into the fabric in film form.
Briscoe, B. J., Motamedi, F., “Role of interfacial friction and lubrication in yarn and fabric mechanics”, TextileResearch Journal 1990 6(12), 697 and Briscoe, B. J., Motamedi, F. “The ballistic impact characteristics of aramid fabrics: the influence of interface friction”, Wear 1992 158(1-2), 229 both describe medium viscosity polymer fluids that were impregnated into fabrics. Additives had a low Tg of −115° C. They found a lubrication effect as expected.
International application (WO 2004/074761 A1) discloses visco-elastic polymer fluids that were solvent impregnated into ballistic fabrics and other related fiber containing ballistic sheets. Preferred range of glass transition temperature (Tg) is −128° C. to −40° C. Low viscosities of 0.25 Pa s to 2.5×104 Pa s were considered.
WO 00/46303 and U.S. Pat. No. 3,649,426 describe polyaramid fabrics with shear-thickening particle suspensions in pouches or in back of polyaramid panels.
Lee, Y. S. et al. (N.J. Advanced Body Armor Utilizing Shear Thickening Fluids, 23rd Army Science Conference, 2002) consider shear-thickening suspensions of particles in conjunction with ballistic fibers.
U.S. Pat. Nos. 5,354,605 and 4,623,574 used low Tg, high molecular weight elastomers as adhesive matrix materials for fiber layers. These provided flexibility in unidirectional ballistic layers.
Applying low levels, less than about 3%, of such solid adhesives from the melt is not effective in improving BFD because the resin cannot flow substantially due to high viscosity and therefore the fabric is incompletely and sparsely impregnated. Applying moderate levels of solid adhesives from the melt is effective in increasing fabric stiffness and thus improving BFD, but the V50 drops substantially and comfort is sacrificed. The phrase “from the melt” means the adhesive can be an originally solid film melted into the fabric surface by laminating or could be extrusion of a thin solvent-free layer of hot polymer from a slit die onto the fabric surface. In both cases, the polymer can get stuck on the outside of the fabric surface and cannot penetrate enough to be effective.
Applying low levels of solid adhesives or elastomers from solution is not effective because the thin adhesive junctions between bundles in the fabric are brittle and cannot heal after mechanical deformation during normal wear.